DORMANCY IMPLICATIONS OF PHOSPHORUS LEVELS IN DEVELOPING CARYOPSES OFWILD OATS (AVENA-FATUA L)

The element phosphorus made up 0.5% of the dry weight of dehulled Aven a fatua caryopses 7 days after anthesis (DAA), half of it inorganic (P -i). Caryopses detached and pierced 7 DAA germinated in vitro with a r apid drop in P-i levels. By 15-20 DAA caryopsis dry weight had increas ed three- to fourfold, but phosphorus made up less than 0.04% of the d ry weight of this enlarged caryopsis. Caryopses at this stage germinat ed readily without piercing if incubated in vitro. A further decrease in P-i accompanied by a marked increase in phytate phosphorus began ab out 15 DAA and continued during later seed maturation. By 20 DAA, when embryos were relatively mature and endosperm cell division had ceased , a decrease in caryopsis water content (as a percentage of dry weight ) began, and seed dormancy became apparent. As starch and phytate rese rves accumulated, P-i and water levels of the caryopsis diminished. Hi gher levels of endogenous P-i coincided with the anabolic events of in itial seed formation and, to a lesser extent, with anabolic events of seed germination. Decreasing P-i levels coincided with accumulation of nutrient reserves, lowering of water content, and the initiation of d ormancy. The data suggest that (1) enzymes associated with the formati on and development of the embryo may be activated by the high P-i leve ls present during initial seed differentiation; (2) embryo quiescence and dormancy are facilitated by the drop of P-i levels which accompani es the accumulation of starch and phytate reserves; and (3) the increa se in P-i which accompanies seed afterripening aids in the termination of dormancy and the resumption of germination.